In many cases a network element comprises one or more ingress line interfaces for receiving data from data transfer links of a data transfer network, one or more egress line interfaces for transmitting data to data transfer links of the data transfer network, and a switch device for transferring data from the ingress line interfaces to the egress line interfaces in accordance with control information associated with the data. The network element can be for example an internet protocol “IP” router, an Ethernet switch, an Asynchronous Transfer Mode “ATM” switch, and/or a Multi-Protocol Label Switching “MPLS” switch. Each data transfer link of the data transfer network can be for example a fiber link, a copper link, or a radio link. A commonly used construction of a network element of the kind described above is such that the network element comprises a frame and plug-in units which are installed in plug-in unit slots of the frame. Electrical or optical connectors in a plug-in unit make galvanic or optical contacts with corresponding electrical or optical connectors in the frame when the plug-in unit is inserted in the plug-in unit slot of the frame. One or more of the plug-in units may constitute the above-mentioned switch device, and other ones of the plug-in units can be line interface modules which comprise the above-mentioned ingress line interfaces and egress line interfaces. Furthermore, there can be one or more plug-in units which constitute a control and/or monitoring device and/or one or more plug-in units which constitute a power supply device.
Network elements of the kind described above should not constitute bottle necks of data transfer in order that the capacity of the data transfer links between the network elements could be effectively utilized. This requirement can be fulfilled when the data transfer capacity from the ingress line interfaces of a network element to the egress line interfaces of the network element is sufficiently high with respect to the data traffic load arriving at the ingress line interfaces and the queuing of data takes place in front of the egress line interfaces. The inherent drawback of the queuing at the egress line interfaces is that also such data which is discarded by the queue management and thereby not forwarded to the data transfer network consumes the above-mentioned data transfer capacity from the ingress line interfaces to the egress line interfaces. The virtual output queuing “VOQ” is a known technique for remedying the above-mentioned drawback. In the virtual output queuing, data is arranged to queue not only at the egress line interfaces but also at the ingress line interfaces prior to being transferred by a switch device to the egress line interfaces. The situations prevailing at the egress line interfaces are signaled to the ingress line interfaces so that the queue management is able to de-queue data from an appropriate queue at an appropriate ingress line interface and to allow the switch device to transfer the data to an appropriate egress line interface when there is room for the data in the buffer of the egress line interface under consideration. In a case of congestion, data is discarded at an ingress line interface. Thus, the data transfer capacity from the ingress line interfaces to egress line interfaces is not wasted on data that will be discarded. On the other hand, the virtual output queuing corresponds functionally to the real output queuing where the queue management is run at the egress line interfaces.
The virtual output queuing of the kind described above is, however, not free from challenges. Some of the challenges are related to the need to signal the queuing situations prevailing at the egress line interfaces to the ingress line interfaces. Especially in cases where the number of ingress line interfaces and/or the number egress line interfaces is/are high, the signaling is complex and it may represent a significant overhead in the data transfer between the ingress and egress line interfaces.